What has shut off rendering and display of the world background?
Switch to perspective view mode in the viewport (Numpad 5). When in orthographic view, which is what you are in now, the world background cannot be properly displayed. Instead, you get a single color based on (I think) the color of the pixel that you are facing. If you have a background with more colors, you will see the color change as you rotate around. Since your background is almost all black, you likely won't see any color change as you rotate your view.